Reconstruction of virtual neural circuits in an insect brain

Abstract

The reconstruction of large-scale nervous systems represents a major scientific and engineering challenge in current neuroscience research that needs to be resolved in order to understand the emergent properties of such systems. We focus on insect nervous systems because they represent a good compromise between architectural simplicity and the ability to generate a rich behavioral repertoire. In insects, several sensory maps have been reconstructed so far. We provide an overview over this work including our reconstruction of population activity in the primary olfactory network, the antennal lobe. Our reconstruction approach, that also provides functional connectivity data, will be refined and extended to allow the building of larger scale neural circuits up to entire insect brains, from sensory input to motor output.

Keywords: digital atlas; identifiable neurons; neuron database; virtual brain.

Figure 1

Reconstruction of the population activity of the antennal lobe projection neurons. Data were obtained by in vivo intracellular recording and staining with glass microelectrodes. Odor-evoked firing activity is monitored (A), averaged across trials, and mean firing rate for each time window is calculated (B). Row and column mean time and glomeruli innervated by the stained projection neurons, respectively. Colors in the figure represent mean firing rate change for each projection neuron in response to cis-3-hexen-1-ol. Scale on the vertical axis represents the time period of odor presentation (500 ms). Dye-filled neurons are imaged by confocal laser scanning microscopy (C), and the glomeruli innervated by the stained neurons are identified (D). Pooled data are integrated with geometrical information using a digital atlas (E). Reconstructed odor-evoked activity by a virtual projection neuron population in response to cis-3-hexan-1-ol is shown. Upper and lower images show anterior and posterior views of the reconstructed dynamics. The number at the bottom is the time elapsed after the onset of odor presentation. Five time points are shown. D, dorsal; L, lateral; M, medial.

Figure 2

Temporal evolution of olfactory representations. By applying dimensionality reduction techniques, odor-evoked activities among projection neuron (PN) population are mapped into a 2D space (A). Illustration of the trajectories representing PN population activity over time in response to cis-3-hexen-1-ol (green), linalool (blue), and citral (red). For each odor, 47 PN spike sequences were divided into consecutive 50-ms time bins and the number of spikes in each bin was counted. We viewed the 47-dimensional vector as the odor representation by the PN population. To visualize the 47-dimensional vector, we applied principal component analysis to the vector and used the first two principal components for the graph. Each representation evolved in an odor-specific manner. (B) Each odor is tested three times. Euclidean distances are calculated for each odor pair. Gray lines represent individual trials and the red line represents the average. Gray box shows odor presentation period (500 ms). PC, principal component.

Figure 3

Spatio-temporal organization of the glomerular network. (A) Functional connectivity as a function of anatomical distance of glomeruli innervated by a pair of projection neurons (PNs). We used Pearson correlation coefficients (r) of the responses of pairs of PNs to quantify functional connectivity. Firing rates from stimulus onset to 1 s after stimulus onset (20 time bins) were used. Pairs of PNs innervating different glomeruli show uncorrelated activity (r = −0.0556, n = 496). (B) A graph of functional connectivity mapped onto x–z plane of the antennal lobe. Each node represents a glomerulus. Only strong connectivity is shown, blue and red lines represent positive (>0.5) and negative (less than −0.5) correlations, respectively. There are many functional connections among glomeruli in the superficial part of the antennal lobe. Although sampling was somewhat sparse, there is a tendency that strong functional connections are rare among deep glomeruli in response to the tested set of plant odors.